The present invention relates to a liquid crystal display device, and more particularly, to an improved liquid crystal display device in which the image retention phenomenon is prevented from occurring on a display image plane.
A liquid crystal display device having a thin film transistor (TFT) comprises a pair of glass substrates with a liquid crystal sealed therebetween. In the liquid crystal display device of this type, a desired image is displayed on an image plane by driving and aligning the liquid crystal in the particular pixel portion in a predetermined direction, thereby transmitting or cutting off the backlight. In such a type of liquid crystal display device, the pixel electrodes and the TFTs are formed on one of the pair of the glass substrates, and generally, a color filter is formed on the other glass substrate. Furthermore, an alignment film made of a resin material such as polyimide or PVA, or an inorganic material is formed on each of the glass substrates. The alignment film is generally provided at a thickness of about 80 nm, and is subjected to rubbing treatment after forming it on the substrate by means of a printing process and the like.
However, a problem of image retention phenomenon is found to occur in a conventional liquid crystal display device having a TFT as described above. That is, in case the display is instantaneously switched off after a given display pattern is displayed continuously for a certain duration of time, the display pattern is found to remain on the image plane even after the display is switched off. The TFTs are driven by alternating current (AC), however, when a still image plane is displayed for a long duration of time, the direct current (DC) component accumulates to disable the liquid crystal shutter. This malfunction occurs assumably due to the charge accumulated in the alignment film. That is, the charge in the alignment film adsorbs ions from inside the liquid crystal that is filled between the two glass substrates and forms an electric double layer.
The image retention phenomena in a conventional liquid crystal display device is illustrated in FIGS. 2 (A) to 2 (C). Referring to FIG. 2 (A), a first image pattern 2 is continuously displayed in a liquid crystal display image plane 1 for a predetermined duration of time. As is shown in FIG. 2 (B), when the image plane 1 is switched off, the afterimage of the first pattern remains as a retained image pattern 3 in the display image plane 1. When a new image pattern 4 is displayed, as is shown in FIG. 2 (C), the new image pattern is displayed on the still remaining retained image pattern 3 to provide an unclear display. Thus, it can be readily understood that the image quality is greatly impaired.
The present invention has been accomplished in the light of the aforementioned circumstances. Accordingly, an object of the present invention is to provide a liquid crystal display device having TFTs, said liquid crystal display device characterized in that the image retention phenomenon is prevented from occurring.